Brainstorm

The feature I desire very much was the ability to play an audio file while I am on call with my friend.

The usecase is automatic voice responder. When someone call me, the app should pick the call and plays some audio files based on the rules preset. Using this app I can tell the callers that "I am busy now" or "call me later" or "number not available (this is for telecallers promotion calls)"

This will be cool. It seems recaller app does the same in the other way. It records all the system audio. I think it may also be possible to play audio to the voice call.